Silver halide photographic materials now commercially available and processes for forming images using them range widely, and are used in many fields. In many cases, the halogen compositions of the silver halide emulsions used in these many photographic materials are silver bromoiodide, silver bromochloroiodide, or silver chlorobromide, mainly comprising silver bromide to attain high sensitivity.
On the other hand, in products used in the market where there is strong demand for a large amount of prints to be finished in a short delivery time, such as photographic materials for color papers, silver bromide or silver chlorobromide substantially free from silver iodide is used due to the need to increase the speed of development.
In recent years there has been increasing demand to improve the rapid processability of color papers, and many studies are being carried out. It is well known that when the content of silver chloride of the silver halide emulsion used is increased, the rate of development is greatly improved.
On the other hand, in the conventional production of silver halide photographic materials, a technique is well known in which a sensitizing dye is added to a silver halide emulsion, to optically sensitize it and expand its photosensitive wavelength range.
Many compounds are conventionally known as spectrally sensitizing dyes used for such purpose, for example cyanine dyes, merocyanine dyes, xanthene dyes, etc., described by T. H. James in The Theory of the Photographic Process, Vol. 3 (1966, Macmillan, N.Y.), pages 198 to 228.
Generally when these sensitizing dyes are applied to silver halide emulsions, in addition to expanding the photosensitive wavelength range, they must also satisfy the following conditions:
(1) The spectrally sensitized range is proper. PA1 (2) The sensitization efficiency is good, and sufficiently high sensitivity can be obtained. PA1 (3) Fogging should not be brought about. PA1 (4) It is narrow in scattering of the sensitivity due to a change of temperature during exposure, due to the interval from exposure to light to processing, or due to the passage of time before use. PA1 (5) It must be free from any adverse interaction with other additives, such as stabilizers, antifoggants, coating aids, and couplers. PA1 (6) When the silver halide emulsion containing the sensitizing dye is stored, in particular at a high temperature under high humidity, the sensitivity must not drop. PA1 (7) Turbidity of colors (mixing of colors) after the development processing due to diffusion of the added sensitizing dye into other photosensitive layers must not take place.
The above conditions have important significance, particularly in the preparation of red-sensitive silver halide emulsions in silver halide color photographic materials.
However, when a silver halide emulsion high in silver chloride content is spectrally sensitized with a red-sensitive sensitizing dye, there are such defects that high sensitivity is not well obtained, and there is a large change in photographic performance due to the passage of time or a change n the interval from exposure to light to processing, which defects have constituted a great impediment to the improvement of silver halide photographic materials excellent in rapid processability.
Some techniques have been proposed to overcome the above defects of silver halide emulsions high in silver halide content.
The correlation of the spectrally sensitized sensitivity of high silver chloride emulsions and the reduction potential of the spectrally sensitizing dyes is reported in Photographic Science and Engineering, Vol. 18, pages 475 to 485 (1974), and in The Journal of Photographic Science, Vol. 21, pages 180 to 186 (1973). In order to improve, for example, the prevention of infrared fogging, maintaining quality over time, or the spectral sensitivity of silver halide emulsions containing silver chloride sensitized spectrally with specific red-sensitive sensitizing dyes, JP-B ("JP-B" means examined Japanese patent publication) Nos. 10473/1971 and 42494/1973 suggest adding nitrogen-containing compounds, JP-A ("JP-A" means unexamined published Japanese patent application) No. 5035/1975 suggests adding supersensitizers, JP-A No. 151026/1977 suggests adding water-soluble bromides, JP-A No. 23520/1979 suggests adding iridium, JP-A No. 202436/1985 suggests adding hardening agents, JP-A No. 7629/1983 suggests improving the way of adding spectrally sensitizing dyes, and JP-A No. 225147/1985 suggests using silver chlorobromide that has (100) and (111) planes.
However, even these techniques have not been able to overcome sufficiently the impediment of the above case wherein silver halide emulsions high in silver halide content are spectrally sensitized with red-sensitive sensitizing dyes.
European Patent Application Publication EP313021A discloses a technique for reducing the change of photographic sensitivity due to a change of temperature during exposure to light in the case wherein emulsions high in silver chloride content are spectrally sensitized with red-sensitive sensitizing dyes. However, in accordance with the inventors' study, it is found that, with this technique, the change of photographic sensitivity due to a change of the interval from exposure to light to processing, that is, the latent-image-keeping property, is not satisfactory, which constitute a great impediment in practice.